1. Field of the Invention
This invention relates to a method and an apparatus for measuring magnetic properties of sheet material such as bank notes.
2. Description of Related Art
Such an apparatus is known from DE 40 22 739. The apparatus has at least one measuring head with at least two pole shoes disposed so as to form a measuring gap. The magnetic signals produced by the magnetic properties of the sheet material at the measuring gap are converted into electric signals by a detector on at least one of the pole shoes.
In order to permit in particular magnetic particles with low remanence to be detected with sufficient reliability, one pole shoe is made of permanent-magnetic material and the other pole shoe of soft magnetic material. The magnetic field produced by the material of the permanent-magnetic pole shoe is selected to be so great that the soft magnetic material of the other pole shoe has a suitable magnetic operating point. This measure permits even low magnetic signals at the measuring gap to produce sufficient electric signals on the detector.
In particular bank notes generally have low magnetic particle densities in the area of the printed image and high ones e.g. in the area of the security thread, so that electric signals with low or high amplitudes arise on the detector during measurement of bank notes.